If there are already tablespaces in hot backup mode, then the Oracle database will raise an error (ORA-01146). Also, the following conditions cause an error to be returned by the alter database begin backup command:
One or more datafiles are offline or missing
An RMAN backup is ongoing
Once you have issued the alter database begin backup command successfully, you can proceed to back up your Oracle database. As is the case when you put individual tablespaces in hot backup mode, you will be unable to perform a normal or immediate shutdown on the database after issuing this command. However, you can issue shutdown abort to terminate the instance if that is required (although one would hope this would never be required).
If the instance crashes or you use the shutdown abort command, you need to take the database out of hot backup mode by using the alter database end backup command... and that is a nice segue to the next topic!
The alter database end backup command has been around since Oracle9i Database, and its functionality is enhanced in Oracle Database 10g. Previously, the alter database end backup command could be used only when the database was mounted. In Oracle Database 10g, you can use this command to end backups with the database mounted or the database open—your choice! Here is an example of the use of this command:
SQL> alter database end backup;
A warning will be issued if any datafiles/tablespaces are not in hot backup mode, but the command will complete successfully.
Oracle Database 10g comes with a plethora of improvements (I like that word, plethora!) for RMAN. These include the following:
Using flash recovery area
Using backup copies and fast recovery